Problem with Regex.Replace() while changing a filename

I am having a strange problem and I can't seem to figure it out.

My filename is something like this:

DER 1513016-3.020F.NCF.

I want to be able to change it to:

DER 1513016-3.020H.NCF

Sometimes the filename can be this as well:

DER 1513016-3.020F_NEW.NCF

which would change to:

DER 1513016-3.020H_NEW.NCF

This is my code to do this:

OpenFileDialog fDialog = new OpenFileDialog();
fDialog.Title = "Open";
fDialog.Filter = "NCF files (*.ncf)|*.ncf|All files (*.*)|*.*";
fDialog.InitialDirectory = "C:\\Program Files";
if (fDialog.ShowDialog() == DialogResult.OK)
{
    string newfilename;
    string fileext = Path.GetExtension(fDialog.FileName);

    newfilename = Regex.Replace(fDialog.FileName, "F.NCF", "H.NCF");
    newfilename = Regex.Replace(fDialog.FileName, "F_NEW.NCF", "H_NEW.NCF");
} 

This is where things get weird. The way the code works now, it will NOT change the filename to DER 1513016-3.020H.NCF

If I comment out this line of code:

//newfilename = Regex.Replace(fDialog.FileName, "F_NEW.NCF", "H_NEW.NCF");

it will work fine and the file will now become: DER 1513016-3.020H.NCF

However,开发者_如何学C if I uncomment that line of code, the filename will not change to DER 1513016-3.020H.NCF. It will stay as DER 1513016-3.020F.NCF.

Why is that line of code causing the routine to not change the filename?

---

Use newfilename instead of fDialog.FileName on the second call or the return value of the first one will never be used.

PS: You can use String.Replace instead of Regex.Replace since you're not using any regular expressions. Plus, the dot means "any character" in a regex so you should consider escaping it.